Abstract
Relaxor ferroelectric (RFE) polymers exhibiting narrow hysteresis loops are attractive for a broad range ofpotential applications such as electric energy storage, artificial muscles, electrocaloric cooling, and printable electronics. However,current state-of-the-art RFE polymers are primarily poly(vinylidene fluoride-co-trifluoroethylene-co-X) [P(VDF-TrFE-X)]random terpolymers with X being 1,1-chlorofluoroethylene (CFE) or chlorotrifluoroethylene (CTFE). Potentialdehydrochlorination at elevated temperatures can prevent the melt-processing of these Cl-containing terpolymers. It isdesirable to achieve the RFE behavior for Cl-free terpolymers such as P(VDF-TrFE-HFP), where HFP stands forhexafluoropropylene. Nonetheless, HFP units were mostly excluded from the crystalline structure because of their large size, andthus no RFE behavior was observed when crystallized from the quiescent melt. Intriguingly, mechanical stretching couldeffectively pull the HFP units into the P(VDF-TrFE) crystals, forming nanosized ferroelectric (FE) domains with a strongphysical pinning effect. Consequently, the RFE behavior was observed for the uniaxially stretched P(VDF-TrFE-HFP) film.Thermal annealing above the Curie temperature (ca. 50 °C) without tension led to the return of the normal FE behavior withbroad hysteresis loops. However, thermal annealing above Curie temperature under tension prevented the exclusion of HFPunits from the crystalline structure, and thus relatively stable RFE behavior was achieved. Various characterization techniqueswere utilized to unravel the structure−property relationships for these P(VDF-TrFE-HFP) films. In addition, the RFE behaviorof P(VDF-TrFE-HFP) was compared to those of other terpolymers. This study provides a unique and simple strategy solelybased on film processing to achieve the RFE behavior for P(VDF-TrFE)-based terpolymers.